The measurement of a light power spectrum using a prior art Michelson interferometer will be described with reference to FIG. 1. Incident light 11 to be measured enters a non-polarizing beam splitter 12 where it is divided into reflected light 14 and transmitted light 15 at the reflecting/transmitting face 13 of the non-polarizing beam splitter 12 angled at 45 degrees to the incident radiation direction. The reflected light 14 and transmitted light 15 are directed at a stationary reflector 16 and a moving reflector 17, respectively. The two light beams 18 and 19 reflected from the stationary and moving reflectors 16, 17 enter the reflecting/transmitting face 13 of the non-polarizing beam splitter where they are recombined to interfere with each other, and the resulting interference light rays emerge therefrom as light beams 21 and 22 orthogonal to each other.
The stationary reflector 16 is fixed in position, so that the length L.sub.1 of the optical path is invariable along which the light travels as it is reflected from the non-polarizing beam splitter 12 and further reflected by the stationary reflector 16 back into the non-polarizing beam splitter 12. On the other hand, the moving reflector 17 is moved back and forth by a drive means, not shown, so that the length L.sub.2 of the optical path is continuously variable along which the light transmitted through the non-polarizing beam splitter 12 travels as it is reflected by the stationary reflector 16 back into the non-polarizing beam splitter 12.
One 21 of the interference rays is introduced into a light receiver 23 where it is converted into an electric signal which is in turn passed to a high-pass filter 24 from which a DC (direct current) component is extracted corresponding to a change in intensity of the interference light 21 that occurs as the moving reflector 17 is moved. The resulting DC signal is converted by an A/D converter 25 to a digital signal which may be fast-Fourier transformed at a Fourier transform processor 26. A power spectrum of the incident ray being measured corresponding to the wavelength obtained through the transformation is thus displayed at a display 27.
Incidentally, the non-polarizing beam splitter 12 has so-called polarized light-dependency in that the reflectivity (or transmittance) thereof varies depending upon the polarization state of the incident light 11. This polarized light-dependency poses the problem that incident ray 11 having different polarization states will vary in the level of power spectrum measured even if they have the same power. This will be further discussed below. Let it be that the amplitude and wavelength of the incident light 11 are E.sub.0 and .lambda., respectively; the wave number is k=2.pi./.lambda.; the amplitude reflectivity and the amplitude transmittance of the non-polarizing beam splitter 12 for P polarized light are R.sub.P.sup.1/2 and T.sub.P.sup.1/2, respectively; and the amplitude reflectivity and the amplitude transmittance of the non-polarizing beam splitter 12 for S polarized light are R.sub.s.sup.1/2 and T.sub.s.sup.1/2, respectively, and that the amplitude of the P polarized light component is E.sub.op and the amplitude of the S polarized light component is E.sub.os. Then, the intensity (power) E.sub.o.sup.2 of the incident light 11 is given in the following equation: EQU E.sub.o.sup.2 =E.sub.op.sup.2 +E.sub.os.sup.2 ( 1)
The amplitude E.sub.1p of the P polarized light component out of the interference light 21 entering the light receiver 23 is expressed by the following equation: EQU E.sub.1p =E.sub.op R.sub.p.sup.1/2 (exp (-ikL.sub.1)+exp (-ikL.sub.2))(2)
Similarly, the amplitude E.sub.1s of the S polarized light component out of the interference light 21 is expressed by the following equation: EQU E.sub.1s =E.sub.os R.sub.s.sup.1/2 T.sub.s.sup.1/2 (exp (-ikL.sub.1)+exp (-ikL.sub.2)) (3)
From the equations (2) and (3), there are obtained the following equations (4) and (5) for the powers I.sub.1p and I.sub.1s of the P and S polarized light components, respectively out of the interference light 21: EQU I.sub.1p =.vertline.E.sub.1p .vertline..sup.2 =2(E.sub.op).sup.2 R.sub.p T.sub.p {1+cos (kn(L.sub.1 -L.sub.2))} (4) EQU I.sub.1s =.vertline.E.sub.1s .vertline..sup.2 =2(E.sub.os).sup.2 R.sub.s T.sub.s {1+cos (kn(L.sub.1 -L.sub.2))} (5)
The power (intensity) I.sub.1 of the interference light 21 is: EQU I.sub.1 =I.sub.1P +I.sub.1s ( 6)
It can be seen from the equations (4) and (5) that I.sub.1p and I.sub.1s will vary depending on the polarization state. Specifically, even though the intensity of the incident light 11 is of the same magnitude, R.sub.p T.sub.p in the equation (4) and R.sub.s T.sub.s in the equation (5) are parameters which are variable depending on the polarization state, so that there may be many cases where R.sub.p T.sub.p .noteq.R.sub.s T.sub.s. Consequently, the level of the electric signal output of the light-receiver 23 varies depending on the polarization state. Such variation in electric signal level can be as great as about 3 dB, depending on the polarization characteristics of the non-polarizing beam splitter 12. This can detract from the accurate measurement when the polarization states are indefinite or when the measurement is carried out while changing the polarization states.
Here, in order to indicate the polarization states, assuming that the proportion of the S polarized light component is M whereas that of the P polarized light component is (1-M), the light intensity of the S polarized light component of the incident ray 11 will be E.sub.os.sup.2 =ME.sub.o.sup.2 while the light intensity of the P polarized light component will be E.sub.op .sup.2 =(1-M) E.sub.o.sup.2. Substituting these terms into the equations (4), (5) and (6) provides the following equations (7), (8) and (9), respectively: ##EQU1##
In the equation (9) the factor which is dependent on the polarized light is expressed as S (M) in the right side to simplify the expression with S (M) representing the term shown in the equation (10). It is because of this polarized light-dependent factor S (M) that the output level of the light-receiver 23 varies.
As discussed above, if there are changes in the polarization state of the incident light, the conventional Michelson interferometer may be incapable of accurately measuring the light power spectrum due to the polarized light-dependent factor S (M), resulting in errors in measurements. This imposes a limitation on the use in measuring an incident ray the polarization state of which varies, undesirably leading to inconvenience in practical use.
Accordingly, an object of this invention is to provide a Michelson interferometer capable of accurately measuring a light power spectrum at all times regardless of the polarization state of the incident ray.